Field of the Invention
The invention relates to a pigment composition for preparing pigmented matt coatings, such as matt paints and printing inks. Further, the invention relates to a process for preparing such pigment composition, and to a coating formulation containing such composition. Finally, the invention is directed to a pigmented matt surface of a substrate, and to the use of the pigment compositions disclosed herein for matting substrates.
Technological Background of the Invention
Matting a surface is achieved by selectively roughening or contouring the surface in the micro range. Thus, impinging light is no longer reflected directionally, but diffusely. The height, shape and number of the surface structures determine the degree of gloss of the paint film or printing ink.
Specific matting agents are usually added to the paint or printing ink composition to achieve a mat surface. The usual matting agents are spherical particles with a particle size within a range of from 1 to about 20 μm, as a function of the film layer thickness. In practice, precipitated or pyrogenic silicas or silica gels are predominantly used as matting agents, but also matting agents based on duromers, waxes, or thermoplasts are used. Further known is the suitability of silicates, calcium carbonate, and different kinds of hollow spheres.
EP 1 398 301 B1 discloses matting agents for paints and lacquers based on precipitated silicas having particle sizes (d50) within a range of from 5 to 15 μm and a specific surface (BET) of from 350 to 550 m2/g. The silicas are optionally coated with wax.
DE 195 16 253 A1 discloses matting agents based on spray-dried silica gel. In order to ensure sufficient stability of the aggregates, a binder, such as a layer silicate, pyrogenic silica or organic polymer, is used. The agent has a particle size of from 1 to 20 μm and a specific pore volume of from 0.4 to 2.5 ml/g.
In general, the matting effect increases with the dosage of the matting agent. Larger particles have a greater matting effect, but lead to a less smooth surface, which is undesirable. Further, both higher contents of matting agent and a longer dispersing time result in increasing viscosities of the paint systems, thus creating processing problems. Further, the known matting agents based on silica are difficult to handle because of a strong tendency to dusting, and are therefore disadvantageous.
According to the patent application EP 1 500 3103, these disadvantages are avoided by using pigment agglomerates having a mean size d50 of from 2 μm to 200 μm in the preparation of matt paints and printing inks. The process is characterized in that the pigment agglomerates partially decay during the dispersing, for example, in a bead mill, but no increase of viscosity occurs. A further feature of the process is that the matting agent and the pigment have the same chemical compositions and surface properties. A drawback of the process is that the degree of matting depends significantly on the input of dispersing energy due to the partial decay of the pigment agglomerates. Thus, the matting effect of that process is greatly affected by the dispersing method.
Therefore, there is a need in the art for a matting agent for preparing matt coatings that is little dusting, dispersion-stable, and further reproducibly enables surfaces having a homogeneous matt appearance. Additionally, there is a need for a process for preparing matt pigmented coatings in which the matting agent has the similar composition as the pigment, but the matting effect is not affected by the dispersing method.